Many methods have been used in audience measurement systems for determining the channels or programs to which television receivers, located in statistically sampled households, are tuned. These methods generally involve sensing signals in the video portions of the monitored television receivers. For example, because the local oscillator frequency of a monitored television receiver is dependent upon the channel to which the monitored television receiver is tuned, the output of the local oscillator may be sensed in order to determine the tuned channel. As another example, signal injection systems sequentially inject a signal on the various carrier frequencies to which a monitored television receiver may be tuned. The injection signal is then sensed in order to identify the tuned channel.
In order to sense a video signal, most audience measurement systems, particularly those which have proven reliable enough for practical use, require at least partial disassembly of the monitored television receiver and a direct connection (such as by soldering) to a point in the video circuitry. Such invasive methods are believed to decrease the likelihood that a sampled household will agree to co-operate in a television audience survey. This loss of cooperation, in turn, both increases the costs of operating the survey and decreases the reliability of the data obtained. Hence, there has been a longstanding need in the television audience field for a reliable non-invasive sensor which does not require physical access by an installing technician to the inside of a cabinet or housing of a monitored television receiver.
Non-invasive sensors, which are located adjacent a sampled television receiver and which measure the frequency and phase of vertical and horizontal synchronization signals that are part of the transmitted television program, are known. For example, Leonard, U.S. Pat. No. 3,130,265, discloses an audience measurement method which requires each transmitter in the surveyed broadcast area to have a unique sync phase. However, the control of the phase of all the transmitters is a condition that has proven impossible to establish.
Gall, in U.S. Pat. No. 4,847,685, discloses a system which (i) detects the phase of both vertical and horizontal synchronization signals for all broadcast stations in a monitored broadcast market, (ii) measures the phase of these signals at a sampled receiver, and (iii) compensates for the distances through which the signals travel from the broadcast stations to both a central monitoring site and a sampled receiver. Solar, in U.S. Pat. No. 4,764,808, discloses a system for determining, from a non-invasive measurement of the horizontal sync frequency of a sampled receiver, the color burst frequency of the station being viewed. This measured frequency is compared with a centrally maintained tabulation of the deviation of each station's actual color burst frequency from a standard value in order to determine the station being viewed. However, neither the Solar system nor the Gall system can discriminate among multiple programs originating from a single location. For example, two channels of satellite-distributed programming that originate at the same uplink facility could have the same color burst frequency and therefore be indistinguishable to the systems disclosed in the Solar and Gall patents. Also, the Solar and Gall systems would be unwieldy if a large number of programming sources were to be measured.
Other systems are content-based systems and identify the programs to which television receivers are tuned either by reading ancillary codes embedded in the programs or by extracting patterns from the programs for comparison to a library of reference patterns. Systems which sense embedded ancillary codes are taught, inter alia, in Haselwood, et al., U.S. Pat. No. 4,025,851, the disclosure of which is herein incorporated by reference, and in Keene, U.S. Pat. No. 5,450,122. The use of pattern recognition is disclosed, inter alia, in Kiewit, et al., U.S. Pat. No. 4,697,209, the disclosure of which is herein incorporated by reference.
Content-based systems typically measure alternating currents and are, therefore, more vulnerable to noise as the measurement bandwidth increases. In order to maximize the signal-to-noise ratio, most of these content-based systems use invasive direct connections to audio or video circuitry within a monitored television receiver. By contrast, there are known content-based systems which non-invasively sense embedded ancillary codes where the ancillary codes (or pattern signatures) vary slowly. For example, a system which non-invasively senses an ancillary code embedded in an audio signal is disclosed in Jensen, et al., U.S. Pat. No. 5,450,490 (this system uses a microphone to pick up the audio in which the ancillary code is embedded). Another system, which switches the luminance of sequential lines of a video signal in order to insert an ancillary code and which senses the ancillary code non-invasively, is disclosed in Schober, et al., U.S. Pat. No. 5,404,160. Although the system disclosed in this Schober, et al. patent operates on a video signal, it does so at data rates that would more conventionally be labeled "audio"--for example, at the 15.6 kHz horizontal line frequency of an NTSC signal.
The present invention is directed to a non-invasive sensor which solves one or more of the above noted problems.